Passive wireless sensor systems that employ resonant circuit technology are known. These systems utilize a passive wireless sensor in remote communication with excitation and reader circuitry. Often the wireless sensor is implanted at a specific location, such as within the human body, to detect and report a sensed parameter. The sensed parameter varies the resonant circuit frequency of the wireless sensor. The reader device samples the resonant frequency of the wireless sensor to determine the sensed parameter.
U.S. Pat. No. 4,127,110 by Bullara dislcoses a sensor for measuring brain fluid pressure measurement. U.S. Pat. No. 4,206,762 by Cosman discloses a similar sensor for measuring internal pressure. Specifically, the Cosman patent describes the use of a grid dip system for wirelessly measuring the resonant frequency of the sensor. In addition, the Cosman patent discloses the possibility of a battery powered portable reader device.
Several methods of reading passive wireless sensors have also been described in prior patents. For example, the Cosman patent discloses an external oscillator circuit that uses the implanted sensor for tuning, and a grid dip measurement system for measurement of sensor resonant frequency. U.S. Pat. No. 6,015,386 by Kensey, et al., discloses a reader that excites the passive sensor by transmitting frequency sweeps and uses a phase detector on the transmit signal to detect the time during the sweep where the transmitted frequency matches the resonance frequency of the sensor. U.S. Pat. No. 6,206,835 by Spillman, et al., discloses a medical implant application for reader technology disclosed in U.S. Pat. No. 5,581,248 by Spillman, et al. This reader technology detects a frequency dependent variable impedance loading effect on the reader by the sensor's detected parameter. U.S. Pat. No. 7,432,723 by Ellis, et al., discloses a reader with energizing loops each tuned to and transmitting different frequencies spaced to ensure that the bandwidth of the sensor allows resonant excitation of the sensor. Ellis uses a ring-down response from the appropriate energizing loop to determine the sensor resonant frequency.
Some readers utilize phased-locked-loop (“PLL”) circuitry to lock onto the sensor's resonant frequency. U.S. Pat. No. 7,245,117 by Joy, et al. discloses an active PLL circuit and signal processing circuit that adjusts a transmitting PLL frequency until the received signal phase and the transmitting PLL signal phase match. When this match occurs, the transmitting PLL frequency is equal to the sensor resonant frequency.
PLL circuits may incorporate sample and hold (S/H) functions to sample the input frequency and hold the PLL at a given frequency. PLLs with S/H may be used in a variety of applications. For example, U.S. Pat. No. 4,531,526 by Genest discloses a reader that uses a PLL circuit with a S/H circuit to adjust the transmitted frequency of the reader to match the resonant frequency received from the sensor. This is done to maximize sensor response to the next transmission and measures the decay rate of the sensor resonance amplitude to extract the sensor value. U.S. Pat. No. 4,644,420 by Buchan describes a PLL with a S/H used to sample a tape data stream and maintain an appropriate sampling frequency for evaluation of digital data pulses on the tape. U.S. Pat. No. 5,006,819 by Buchan, et al., provides additional enhancements to this concept. U.S. Pat. No. 5,920,233 by Denny describes a high-speed sampling technique using a S/H circuit with a PLL to reduce the charge pump noise from the phase-frequency detector to enhance the low-jitter performance of a frequency synthesizing circuit. U.S. Pat. No. 4,511,858 by Charvit, et al., discloses a PLL with a S/H circuit to pre-position the control voltage of a voltage controlled oscillator when the PLL lock frequency is being changed. This is done to enhance the response speed of the PLL when changing the desired synthesized frequency. U.S. Pat. No. 6,570,457 by Fischer and U.S. Pat. No. 6,680,654 by Fischer, et al., disclose a PLL with S/H circuitry to enhance PLL frequency stepping as well as offset correction feature. U.S. Pat. No. 3,872,455 by Fuller, et al. discloses a PLL having a digital S/H to freeze the frequency display and preload the frequency counter when a PLL phase lock is detected.
Current designs for passive sensor readers, such as those disclosed above, suffer from a number of deficiencies. Swept frequency sensor readers similar to those described in the Cosman, Kensey, Ellis and Spillman patents require relatively wide bandwidth allowance by the government body regulating radio transmissions. This limits other uses of the spectrum and makes interference a potential issue. Readers that track the resonant frequency of a passive resonant sensor with a variable frequency transmitter, such as Genest, Ellis, and Joy also suffer from similar problems. Moreover, the amount of electrical power needed for transmissions, signal processing, sampling, and tracking the resonant frequency of a sensor using digitally controlled frequency tracking or swept frequency systems is significant and limits the ability to use battery power in a reader, as well as limiting the longevity of batteries in a battery powered reader. Accordingly, an improved passive sensor reader is needed in the art.